// Package structs contains various utilities functions to work with structs.
package structs

import (
	"fmt"

	"reflect"
)

var (
	// DefaultTagName is the default tag name for struct fields which provides
	// a more granular to tweak certain structs. Lookup the necessary functions
	// for more info.
	DefaultTagName = "structs" // struct's field default tag name
)

// Struct encapsulates a struct type to provide several high level functions
// around the struct.
type Struct struct {
	raw     interface{}
	value   reflect.Value
	TagName string
}

// New returns a new *Struct with the struct s. It panics if the s's kind is
// not struct.
func New(s interface{}) *Struct {
	return &Struct{
		raw:     s,
		value:   strctVal(s),
		TagName: DefaultTagName,
	}
}

// Map converts the given struct to a map[string]interface{}, where the keys
// of the map are the field names and the values of the map the associated
// values of the fields. The default key string is the struct field name but
// can be changed in the struct field's tag value. The "structs" key in the
// struct's field tag value is the key name. Example:
//
//   // Field appears in map as key "myName".
//   Name string `structs:"myName"`
//
// A tag value with the content of "-" ignores that particular field. Example:
//
//   // Field is ignored by this package.
//   Field bool `structs:"-"`
//
// A tag value with the content of "string" uses the stringer to get the value. Example:
//
//   // The value will be output of Animal's String() func.
//   // Map will panic if Animal does not implement String().
//   Field *Animal `structs:"field,string"`
//
// A tag value with the option of "omitnested" stops iterating further if the type
// is a struct. Example:
//
//   // Field is not processed further by this package.
//   Field time.Time     `structs:"myName,omitnested"`
//   Field *http.Request `structs:",omitnested"`
//
// A tag value with the option of "omitempty" ignores that particular field if
// the field value is empty. Example:
//
//   // Field appears in map as key "myName", but the field is
//   // skipped if empty.
//   Field string `structs:"myName,omitempty"`
//
//   // Field appears in map as key "Field" (the default), but
//   // the field is skipped if empty.
//   Field string `structs:",omitempty"`
//
// Note that only exported fields of a struct can be accessed, non exported
// fields will be neglected.
func (s *Struct) Map() map[string]interface{} {
	out := make(map[string]interface{})
	s.FillMap(out)
	return out
}

// FillMap is the same as Map. Instead of returning the output, it fills the
// given map.
func (s *Struct) FillMap(out map[string]interface{}) {
	if out == nil {
		return
	}

	fields := s.structFields()

	for _, field := range fields {
		name := field.Name
		val := s.value.FieldByName(name)

		var finalVal interface{}

		tagName, tagOpts := parseTag(field.Tag.Get(s.TagName))
		if tagName != "" {
			name = tagName
		}

		// if the value is a zero value and the field is marked as omitempty do
		// not include
		if tagOpts.Has("omitempty") {
			zero := reflect.Zero(val.Type()).Interface()
			current := val.Interface()

			if reflect.DeepEqual(current, zero) {
				continue
			}
		}

		if IsStruct(val.Interface()) && !tagOpts.Has("omitnested") {
			// look out for embedded structs, and convert them to a
			// map[string]interface{} too
			n := New(val.Interface())
			n.TagName = s.TagName
			m := n.Map()
			if len(m) == 0 {
				finalVal = val.Interface()
			} else {
				finalVal = m
			}
		} else {
			finalVal = val.Interface()
		}

		if tagOpts.Has("string") {
			s, ok := val.Interface().(fmt.Stringer)
			if ok {
				out[name] = s.String()
			}
			continue
		}

		out[name] = finalVal
	}
}

// Values converts the given s struct's field values to a []interface{}.  A
// struct tag with the content of "-" ignores the that particular field.
// Example:
//
//   // Field is ignored by this package.
//   Field int `structs:"-"`
//
// A value with the option of "omitnested" stops iterating further if the type
// is a struct. Example:
//
//   // Fields is not processed further by this package.
//   Field time.Time     `structs:",omitnested"`
//   Field *http.Request `structs:",omitnested"`
//
// A tag value with the option of "omitempty" ignores that particular field and
// is not added to the values if the field value is empty. Example:
//
//   // Field is skipped if empty
//   Field string `structs:",omitempty"`
//
// Note that only exported fields of a struct can be accessed, non exported
// fields  will be neglected.
func (s *Struct) Values() []interface{} {
	fields := s.structFields()

	var t []interface{}

	for _, field := range fields {
		val := s.value.FieldByName(field.Name)

		_, tagOpts := parseTag(field.Tag.Get(s.TagName))

		// if the value is a zero value and the field is marked as omitempty do
		// not include
		if tagOpts.Has("omitempty") {
			zero := reflect.Zero(val.Type()).Interface()
			current := val.Interface()

			if reflect.DeepEqual(current, zero) {
				continue
			}
		}

		if tagOpts.Has("string") {
			s, ok := val.Interface().(fmt.Stringer)
			if ok {
				t = append(t, s.String())
			}
			continue
		}

		if IsStruct(val.Interface()) && !tagOpts.Has("omitnested") {
			// look out for embedded structs, and convert them to a
			// []interface{} to be added to the final values slice
			for _, embeddedVal := range Values(val.Interface()) {
				t = append(t, embeddedVal)
			}
		} else {
			t = append(t, val.Interface())
		}
	}

	return t
}

// Fields returns a slice of Fields. A struct tag with the content of "-"
// ignores the checking of that particular field. Example:
//
//   // Field is ignored by this package.
//   Field bool `structs:"-"`
//
// It panics if s's kind is not struct.
func (s *Struct) Fields() []*Field {
	return getFields(s.value, s.TagName)
}

// Names returns a slice of field names. A struct tag with the content of "-"
// ignores the checking of that particular field. Example:
//
//   // Field is ignored by this package.
//   Field bool `structs:"-"`
//
// It panics if s's kind is not struct.
func (s *Struct) Names() []string {
	fields := getFields(s.value, s.TagName)

	names := make([]string, len(fields))

	for i, field := range fields {
		names[i] = field.Name()
	}

	return names
}

func getFields(v reflect.Value, tagName string) []*Field {
	if v.Kind() == reflect.Ptr {
		v = v.Elem()
	}

	t := v.Type()

	var fields []*Field

	for i := 0; i < t.NumField(); i++ {
		field := t.Field(i)

		if tag := field.Tag.Get(tagName); tag == "-" {
			continue
		}

		f := &Field{
			field: field,
			value: v.FieldByName(field.Name),
		}

		fields = append(fields, f)

	}

	return fields
}

// Field returns a new Field struct that provides several high level functions
// around a single struct field entity. It panics if the field is not found.
func (s *Struct) Field(name string) *Field {
	f, ok := s.FieldOk(name)
	if !ok {
		panic("field not found")
	}

	return f
}

// Field returns a new Field struct that provides several high level functions
// around a single struct field entity. The boolean returns true if the field
// was found.
func (s *Struct) FieldOk(name string) (*Field, bool) {
	t := s.value.Type()

	field, ok := t.FieldByName(name)
	if !ok {
		return nil, false
	}

	return &Field{
		field:      field,
		value:      s.value.FieldByName(name),
		defaultTag: s.TagName,
	}, true
}

// IsZero returns true if all fields in a struct is a zero value (not
// initialized) A struct tag with the content of "-" ignores the checking of
// that particular field. Example:
//
//   // Field is ignored by this package.
//   Field bool `structs:"-"`
//
// A value with the option of "omitnested" stops iterating further if the type
// is a struct. Example:
//
//   // Field is not processed further by this package.
//   Field time.Time     `structs:"myName,omitnested"`
//   Field *http.Request `structs:",omitnested"`
//
// Note that only exported fields of a struct can be accessed, non exported
// fields  will be neglected. It panics if s's kind is not struct.
func (s *Struct) IsZero() bool {
	fields := s.structFields()

	for _, field := range fields {
		val := s.value.FieldByName(field.Name)

		_, tagOpts := parseTag(field.Tag.Get(s.TagName))

		if IsStruct(val.Interface()) && !tagOpts.Has("omitnested") {
			ok := IsZero(val.Interface())
			if !ok {
				return false
			}

			continue
		}

		// zero value of the given field, such as "" for string, 0 for int
		zero := reflect.Zero(val.Type()).Interface()

		//  current value of the given field
		current := val.Interface()

		if !reflect.DeepEqual(current, zero) {
			return false
		}
	}

	return true
}

// HasZero returns true if a field in a struct is not initialized (zero value).
// A struct tag with the content of "-" ignores the checking of that particular
// field. Example:
//
//   // Field is ignored by this package.
//   Field bool `structs:"-"`
//
// A value with the option of "omitnested" stops iterating further if the type
// is a struct. Example:
//
//   // Field is not processed further by this package.
//   Field time.Time     `structs:"myName,omitnested"`
//   Field *http.Request `structs:",omitnested"`
//
// Note that only exported fields of a struct can be accessed, non exported
// fields  will be neglected. It panics if s's kind is not struct.
func (s *Struct) HasZero() bool {
	fields := s.structFields()

	for _, field := range fields {
		val := s.value.FieldByName(field.Name)

		_, tagOpts := parseTag(field.Tag.Get(s.TagName))

		if IsStruct(val.Interface()) && !tagOpts.Has("omitnested") {
			ok := HasZero(val.Interface())
			if ok {
				return true
			}

			continue
		}

		// zero value of the given field, such as "" for string, 0 for int
		zero := reflect.Zero(val.Type()).Interface()

		//  current value of the given field
		current := val.Interface()

		if reflect.DeepEqual(current, zero) {
			return true
		}
	}

	return false
}

// Name returns the structs's type name within its package. For more info refer
// to Name() function.
func (s *Struct) Name() string {
	return s.value.Type().Name()
}

// structFields returns the exported struct fields for a given s struct. This
// is a convenient helper method to avoid duplicate code in some of the
// functions.
func (s *Struct) structFields() []reflect.StructField {
	t := s.value.Type()

	var f []reflect.StructField

	for i := 0; i < t.NumField(); i++ {
		field := t.Field(i)
		// we can't access the value of unexported fields
		if field.PkgPath != "" {
			continue
		}

		// don't check if it's omitted
		if tag := field.Tag.Get(s.TagName); tag == "-" {
			continue
		}

		f = append(f, field)
	}

	return f
}

func strctVal(s interface{}) reflect.Value {
	v := reflect.ValueOf(s)

	// if pointer get the underlying element≤
	if v.Kind() == reflect.Ptr {
		v = v.Elem()
	}

	if v.Kind() != reflect.Struct {
		panic("not struct")
	}

	return v
}

// Map converts the given struct to a map[string]interface{}. For more info
// refer to Struct types Map() method. It panics if s's kind is not struct.
func Map(s interface{}) map[string]interface{} {
	return New(s).Map()
}

// FillMap is the same as Map. Instead of returning the output, it fills the
// given map.
func FillMap(s interface{}, out map[string]interface{}) {
	New(s).FillMap(out)
}

// Values converts the given struct to a []interface{}. For more info refer to
// Struct types Values() method.  It panics if s's kind is not struct.
func Values(s interface{}) []interface{} {
	return New(s).Values()
}

// Fields returns a slice of *Field. For more info refer to Struct types
// Fields() method.  It panics if s's kind is not struct.
func Fields(s interface{}) []*Field {
	return New(s).Fields()
}

// Names returns a slice of field names. For more info refer to Struct types
// Names() method.  It panics if s's kind is not struct.
func Names(s interface{}) []string {
	return New(s).Names()
}

// IsZero returns true if all fields is equal to a zero value. For more info
// refer to Struct types IsZero() method.  It panics if s's kind is not struct.
func IsZero(s interface{}) bool {
	return New(s).IsZero()
}

// HasZero returns true if any field is equal to a zero value. For more info
// refer to Struct types HasZero() method.  It panics if s's kind is not struct.
func HasZero(s interface{}) bool {
	return New(s).HasZero()
}

// IsStruct returns true if the given variable is a struct or a pointer to
// struct.
func IsStruct(s interface{}) bool {
	v := reflect.ValueOf(s)
	if v.Kind() == reflect.Ptr {
		v = v.Elem()
	}

	// uninitialized zero value of a struct
	if v.Kind() == reflect.Invalid {
		return false
	}

	return v.Kind() == reflect.Struct
}

// Name returns the structs's type name within its package. It returns an
// empty string for unnamed types. It panics if s's kind is not struct.
func Name(s interface{}) string {
	return New(s).Name()
}